Free-space optical (FSO) communications can enable high-speed wireless communications over a sizable range (e.g., many kilometers). In terrestrial applications, FSO communications can achieve very high (e.g., more than 10 Gbps) data rates without the need to lay fiber-optic cable. Unlike communications over fiber-optic environment, FSO communications has to deal with atmospheric turbulence, which causes substantial phase and amplitude fluctuations of the FSO signal.
One approach for an FSO communications system to mitigate the phase effects is to use direct detect encoding schemes, such as an on-off-keying (OOK), paired with relatively large high-rate avalanche photo-diodes (APDs). An APD can be coupled to a multi-mode fiber (MMF) with a relatively large (e.g., larger than 50 μm) fiber core diameter. The large fiber core of a MMF allows an FSO terminal to become relatively less sensitive to phase effects in the aperture (e.g., for relatively small apertures) than for systems that use single-mode fiber (SMF). The MMF does not lend itself to the types of network architectures that have been implemented in telecommunications applications using wavelength-division multiplexing (WDM) and optical add-drop modules (OADMs). The WDM and OADM devices are built for single-mode fibers (SMFs), which are not appropriate for direct-detect FSO communication system. This is because using the SMFs in FSO communication systems based on direct-detect schemes would introduce a substantial (e.g., 15 to 20 dB) loss to the system.